[Devel] [RFC][PATCH 3/4]: Enable multiple mounts of /dev/pts

[Pavel Emelyanov]

sukadev at us.ibm.com wrote:
> Pavel Emelianov [xemul at openvz.org] wrote:
> | Serge E. Hallyn wrote:
> | > Quoting Pavel Emelyanov (xemul at openvz.org):
> | >> [snip]
> | >>
> | >>>> Mmm. I wanted to send one small objection to Cedric's patches with mqns,
> | >>>> but the thread was abandoned by the time I decided to do-it-right-now.
> | >>>>
> | >>>> So I can put it here: forcing the CLONE_NEWNS is not very good, since
> | >>>> this makes impossible to push a bind mount inside a new namespace, which
> | >>>> may operate in some chroot environment. But this ability is heavily
> | >>> Which direction do you want to go?  I'm wondering whether mounts
> | >>> propagation can address it.
> | >> Hardly. AFAIS there's no way to let the chroot-ed tasks see parts of
> | >> vfs tree, that left behind them after chroot, unless they are in the 
> | >> same mntns as you, and you bind mount this parts to their tree. No?
> | > 
> | > Well no, but I suspect I'm just not understanding what you want to do.
> | > But if the chroot is under /jail1, and you've done, say,
> | > 
> | > 	mkdir -p /share/pts
> | > 	mkdir -p /jail1/share
> | > 	mount --bind /share /share
> | > 	mount --make-shared /share
> | > 	mount --bind /share /jail1/share
> | > 	mount --make-slave /jail1/share
> | > 
> | > before the chroot-ed tasks were cloned with CLONE_NEWNS, then when you
> | > do
> | > 
> | > 	mount --bind /dev/pts /share/pts
> | > 
> | > from the parent mntns (not that I know why you'd want to do *that* :)
> | > then the chroot'ed tasks will see the original mntns's /dev/pts under
> | > /jail1/share.
> | 
> | I haven't yet tried that, but :( this function
> | 
> | 	static inline int check_mnt(struct vfsmount *mnt)
> | 	{
> | 	        return mnt->mnt_ns == current->nsproxy->mnt_ns;
> | 	}
> | 
> | and this code in do_loopback
> | 
> |         if (!check_mnt(nd->mnt) || !check_mnt(old_nd.mnt))
> |                 goto out;
> | 
> | makes me think that trying to bind a mount from another mntns
> | ot _to_ another is prohibited... Do I miss something?
> | 
> | >>> Though really, I think you're right - we shouldn't break the kernel
> | >>> doing CLONE_NEWMQ or CLONE_NEWPTS without CLONE_NEWNS, so we shouldn't
> | >>> force the combination.
> | >>>
> | >>>> exploited in OpenVZ, so if we can somehow avoid forcing the NEWNS flag
> | >>>> that would be very very good :) See my next comment about this issue.
> | >>>>
> | >>>>> Pavel, not long ago you said you were starting to look at tty and pty
> | >>>>> stuff - did you have any different ideas on devpts virtualization, or
> | >>>>> are you ok with this minus your comments thus far?
> | >>>> I have a similar idea of how to implement this, but I didn't thought
> | >>>> about the details. As far as this issue is concerned, I see no reasons
> | >>>> why we need a kern_mount-ed devtpsfs instance. If we don't make such,
> | >>>> we may safely hold the ptsns from the superblock and be happy. The
> | >>>> same seems applicable to the mqns, no?
> | >>> But the current->nsproxy->devpts->mnt is used in several functions in
> | >>> patch 3.
> | >> Indeed. I overlooked this. Then we're in a deep ... problem here.
> | >>
> | >> Breaking this circle was not that easy with pid namespaces, so
> | >> I put the strut in proc_flush_task - when the last task from the
> | >> namespace exits the kern-mount-ed vfsmnt is dropped, but we can't
> | >> do the same stuff with devpts.
> | > 
> | > But I still don't see what the problem is with my proposal?  So long as
> | > you agree that if there are no tasks remaining in the devptsns,
> | > then any task which has its devpts mounted should see an empty directory
> | > (due to sb->s_info being NULL), I think it works.
> | 
> | Well, if we _do_ can handle the races with ns->devpts_mnt switch
> | from not NULL to NULL, then I'm fine with this approach.
> | 
> | I just remember, that with pid namespaces this caused a complicated
> | locking and performance degradation. This is the problem I couldn't
> | remember yesterday.
> 
> Well, iirc, one problem with pid namespaces was that we need to keep
> the task and pid_namespace association until the task was waited on
> (for instance the wait() call from parent needs the pid_t of the
> child which is tied to the pid ns in struct upid).
> 
> For this reason, we don't drop the mnt reference in free_pid_ns() but
> hold the reference till proc_flush_task().
> 
> With devpts, can't we simply drop the reference in free_pts_ns() so
> that when the last task using the pts_ns exits, we can unmount and
> release the mnt ?

I hope we can. The thing I'm worried about is whether we can correctly
handle race with this pointer switch from NULL to not-NULL.

> IOW, do you suspect that the circular reference leads to leaking vfsmnts ?
> 

Of course! If the namespace holds the vfsmnt, vfsmnt holds the superblock
and the superblock holds the namespace we won't drop this chain ever,
unless some other object breaks this chain.
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